The ability to nodularize cast iron was significantly advanced some 27 years ago when it became known that magnesium, rare earth metals, calcium or their alloys (hereinafter referred to as the alloy), will reliably condition a molten iron charged to form nodular graphite upon solidification. Since that time, the art has moved progressively from (a) adding the alloy to the molten iron charge in the ladle by such methods as plunging, immersion or the sandwich technique to (b) adding the alloy to the molten charge in a stream immediately before entering the mold, and finally to (c) adding the alloy into a portion of the gating system within the mold.
The earliest use of adding the alloy to a portion of the gating system in the mold was developed particularly with respect to inoculation, a form of cast iron and nodular iron conditioning which not only heralded the way but proved that total nodularization can be carried out within the mold. All of the in-the-mold techniques have possessed one common characteristic, namely: the alloy has been introduced in a particulate or powdered form or a compact made of these. The particulate alloy was (1) introduced in measured scoops spilled into a reaction chamber defined in a sand mold or (2) the alloy was premolded in particulate form within a foam suspension defining the gating system, or (3) a precompacted or extruded shape of particulate magnesium alloy was placed in the gating system contacting only one supporting surface. The latter has only been conceptually brought forth; it has not been used in a practical manner to date.
This progression of technology has resulted in a more matched use of magnesium or other nodularizing agent with the needs of the specific casting, it has eliminated fading effects associated with the use of the alloy, eliminated flare and other environmental problems, and has aided in reducing costs. Nonetheless, there still remains the likelihood of (a) defects in the casting resulting from undissolved or nonuniformly mixed particulate nodularizing agent which has floated or has been carried into the cavity, (b) variable segregation of the alloy or a variable solubility rate causing a metallurgical variation in the casting, (c) unnecessary waste resulting from expansion of the volume of the gating system to accommodate the particulate matter, (d) the inability to closely target the minimum amount of magnesium to obtain complete or partial nodularization, (e) slag defects in the casting resulting from the greater surface oxidation of the selected nodularizing agent used in particulate form, (f) the inability to remove the alloy from unpoured molds, thus deteriorating the molding properties of the sand mixture in said unpoured molds.
Even if the nodularizing agent was used in a very elemental cast form, prior to its being ground and sized into a particulate or powder form, such cast form would not achieve the objects of this invention because (a) it is not in a condition which will fit the variety of sizes and quantities required of different casting applications without special tailoring a specific such application, (b) the cast form usually is not made and therefore cannot be later converted to an angular form which may be required for a predetermined solution rate, and (c) the cast form generally has not been able to be made in thicknesses greater than 1.25 inches without encountering significant segregation within the interior of the cast form.